The evolutionary stage of a galaxy is primarily given by its star formation history and where within a galaxy the stars form. Both of these quantities depend on the physical conditions of the star-forming interstellar medium. Stellar evolution, i.e. radiation, winds, supernovae, will in turn modify the ISM by injecting energy and newly processed material into the ISM thereby enriching it with heavy elements. This interplay of stars and the ISM thus defines the evolutionary stage of a galaxy.

Stellar radiation heats the dust and is then reradiated at far-infrared wavelengths. In contrast to the UV traced e.g. by the recent GALEX observations, the FIR continuum is an almost extinction free tracer of the stellar radiation. The gas is primarily heated via photo-electric effect on dust grains and cooled via the forbidden [CII] 158µm micron and [OI] 63µm fine-structure lines. These contribute up to 1% to the total FIR intensity (Contursi et al. 2002, Malhotra et al. 2001, Stacey et al. 1991). While [OI] traces solely the neutral gas, a significant part of the [CII] emission stems from the diffuse and dense ionized medium. Separating the relative importance and the properties of the phases of the interstellar medium by spectrally resolved observations is one of the major topics of the envisaged observational projects to be conducted with SOFIA and Herschel. Other important cooling lines, namely the rotational lines of CO and the two fine structure lines of [CI], and also of [NII] at 205 and 122 µm, are needed to address this question. Due to their different critical densities and upper energy levels, they trace different regimes of the ISM allowing to sample gas at different temperatures and densities. The ratio of the two [CI] lines allows for example to estimate the kinetic temperatures of the [CI] emitting region while the ratio of the two [NII] lines allows to estimate the relative importance of the two ionized phases. Line ratios of two CO transitions or of e.g. CO 7-6 vs. CI 2-1 are very sensitive tracers of the densities. Also, the ratio of [CI]/[CII] is known to be a good tracer of G0, the strength of the interstellar FUV radiation field.

We identified three major objectives: the energy balance of gas cooling and heating, the impact of sub-solar metallicities, and the phases of the ISM.

1. Addressing the energy balance:

[CII] is an excellent tracer of the star forming activity and, together with the [OI] 63 micron line, one of the major cooling lines of the ISM as has been seen by KAO (Stacey et al. 1991) and ISO/LWS observations (Malhotra et al. 2001). [CII] and FIR dust emisison are usually tightly correlated. The fraction of the two major cooling lines over the FIR is a measure of the gas heating efficiency. ISO observations show that it varies between 1% and less than 0.05% for normal galaxies. Malhotra et al. find a trend of reduced photoelectric heating efficiency with rising FUV field, presumably due to increased grain charge, but self-absorption of [OI](63μm) but also of [CII] may become important (Stacey). Velocity resolved observations with SOFIA and Herschel will address this question. The scale height of the tracers and phases shall be studied in edge-on galaxies. The FUV radiation of young, massive stars impinge on the surrounding dusty clouds, heating the dust and also the gas, thereby creating photon dominated regions (PDRs). Cosmic Rays are known to be important to heat the dense, shielded cloud core interiors at ~10K. However, in the centers of galaxies like NGC253 (Bradford et al. 2003), the supernova rate is more than 2 orders of magnitude higher than in the Milky Way. Cosmic Rays may therefore dominate the heating in this environment. Another source of energy input are shocks created by massive stars or AGNs. The elevated abundance of SiO (Fuente) and the highly excited CO lines (Bradford) in NGC253 indicate for example that shocks play an important role.

MIR emission at 7 and 15 micron traces transiently heated PAHs and VSGs and has been mapped by ISOCAM in a large scample of galaxies (e.g. Schreiber et al. 2003). Since the smallest particles are most important for the photo-electrical heating in PDRs, these data, in principle, allows to constrain PDR models (Contursi et al. 2004). UV images of galaxies have recently been created with GALEX (Boissier et al. in prep.). Taking into account UV extinction in conjunction with FIR, Hα, and Hβ data, allows to constrain PDR models and to determine e.g. the star forming rate. Additional constraints on the energy balance in galaxies come from the ongoing SINGS nearby galaxies survey with Spitzer (Kennicutt et al. 2003). Recent IRAC images of M51 reveal new spokelike structures bridging the gaps between the spiral arms. Of particular importance will be the Spitzer data archive of ancillary data which will include BIMA SONG CO and HI aperture synthesis data.

2. The importance of metallicities:

Subsolar metallicities at large galacto-centric distance, in the halo of spirals, and possibly also in the interarm medium (and in dwarfs studied in C1), as also super-solar metallicities observed in starbursts, have a strong impact on the energy balance of the ISM (Roellig et al. 2005a). Low metallicities may lead to the photo-destruction of CO to much greater depths. Under these conditions, [CI] and [CII] may be the best tracers of molecular clouds. While CO and [CI] originate in molecular clouds, [CII] stems partly from the ionized medium, and [NII] is solely emitted from the ionized phases. Measurement of the relative elemental abundances will be important, for example using the [NIII](57μm)/[OIII](52μm) ratio tracing the ionized medium observed by ISO/LWS.

The [NIII](57μm)/[NII](122μm) ratio, also tracing the ionized medium, allows to constrain the hardness of the ambient stellar UV radiation field in order to estimate the spectral type of the OB stars (Stacey et al. 1999).

3. The phases of the ISM:

The different phases of the ISM and their relative importance in the different environments will be addressed by observations of selected line ratios. The kinematic information carried by velocity resolved FIR lines was to date largely unavailable. MHD-models of line emission from the various phases show that the line shapes are sensitive indicators of the emitting phases (Hennebelle & Perault 2000). The few velocity resolved data obtained at the KAO, show the great potential of analyzing these data: See the 13[CII]/12[CII] observations of Boreiko & Betz (1996) and the [NII]122μm/255μm observations of Colgan et al. (1993) of Galactic star forming regions.

The FIR emission of M51 and M83 We have focussed our first effort on the two nearby face-on spirals M83 and M51 at 3.7 and 9.6 Mpc distance. For the first time, we have detected the [CI] 1-0 line at 10" resolution in the spiral armsat galacto-centric distances of upto 5.8 kpc.These data have been combined with complementary low and mid-J CO data at similar resolutions from Garcia-Burillo et al. (1993), Crosthwaite et al. (2002), Dumke et al. (2001). In addition, we have obtained ISO/LWS data of the [CII], [OI](63), and [NII](122) lines and the total infrared intensities from HIRES. Aiming at a detailed comparison with PDR models, we have corrected the data for beam filling and the [CII] data for the contribution from the ionized medium which was found to contribute between 100% and 60% to the observed [CII] emission. The detailed PDR modelling allowed us to derive the local densities and FUV fields, the grain photo electric heating efficiency, volume and area filling factors, etc..

The large-scale distribution of the extended molecular gas in M51 (Figure) is currently being studied with HERA2/IRAM-30m observations in a joined project with K.Schuster, H. Wiesemeyer (IRAM) and S.Garcial-Burillo (Yebes). The 18 pixels of the new HERA2 receiver enables efficient observing of the CO 2-1 line allowing to trace the interarm gas and any extended gas component as seen in HI (Rots et al. 1990). Integrated CO 2-1 intensities of M51 covering 10'x10' at 10" resolution. The map includes the companion galaxy NGC5195 near (+100",+250") and a faint southern spiral arm near (-150",-200"), previously only detected in HI. Some artefacts of the data processing are still visible. The map was obtained in February 2005 with the 18-channel HERA receiver at the IRAM-30m telescope (Schuster, Kramer, Mookerjea, Garcia-Burillo et al. 2005, in prep.).

We contributed to the HIFI/Herschel guaranteed time key project on the "Physical and Chemical Conditions of the ISM in Galactic Nuclei" led by R.Güsten (MPIfR). One important aspect will be to observe the CII, OI, NII, CI, and high-J CO transitions of the inner few square-arcminutes of the nearby moderate luminosity galaxies NGC253 and M83 - using also PACS/Herschel. We will be able to study the inner regions at scales of better than 180 pc, i.e. in much more detail than has so far been possible with the KAO or ISO 50" to 80" beams. Mid-J CO observations conducted from the ground (Petitpas & Wilson 1998, Bradford et al. 2003) indicate a large fraction of warm, dense gas in both nuclei, which is difficult to explain with current PDR models. Other possible heating mechanisms are discussed by e.g. Martin et al. (2003). The proposed observations will allow a much more complete analysis than has so far been possible.